U.S. patent number 5,132,826 [Application Number 07/522,215] was granted by the patent office on 1992-07-21 for ferroelectric liquid crystal tunable filters and color generation.
This patent grant is currently assigned to The University of Colorado Foundation, Inc.. Invention is credited to Kristina M. Johnson, Gary D. Sharp.
United States Patent |
5,132,826 |
Johnson , et al. |
July 21, 1992 |
**Please see images for:
( Certificate of Correction ) ** |
Ferroelectric liquid crystal tunable filters and color
generation
Abstract
Discretely and continuously tunable filers emplying FLC cells
are provided. Exemplary discretely tunable filters employ bistable
smectic C* FLC cells. Exemplary continuously tunable filters employ
smectic A* FLC cells. Single or multiple stage filters are
provided. Blocking filters useful for color generation are also
provided. The FLC filters provided can be temporally
multiplexed.
Inventors: |
Johnson; Kristina M. (Boulder,
CO), Sharp; Gary D. (Boulder, CO) |
Assignee: |
The University of Colorado
Foundation, Inc. (Boulder, CO)
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Family
ID: |
23702670 |
Appl.
No.: |
07/522,215 |
Filed: |
May 11, 1990 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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429304 |
Oct 30, 1989 |
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Current U.S.
Class: |
349/18; 349/117;
349/172; 349/80 |
Current CPC
Class: |
G02F
1/13473 (20130101); G02F 1/141 (20130101); G02F
2203/055 (20130101) |
Current International
Class: |
G02F
1/13 (20060101); G02F 1/141 (20060101); G02F
1/1347 (20060101); G02F 001/13 () |
Field of
Search: |
;350/335,339R,35S,334,337 ;359/53,56,63,66,73,93,94,100 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Other References
Carrington et al. (1989) Second Internaitonal Conference on
Ferroelectric Liquid Crystals Program and Abstracts; Abstract No.
015. .
Andersson et al. (1987) Appl. Phys. Lett vol. 51:640
"Submicrosecond electro-optic switching in LC smectic A phase: The
solf mode ferroelectric effect", Appl. Phys. Lett. vol. 51 No. 9,
1987 pp. 640-642. .
Title et al. "Tunable birefringent networks" SPIE vol. 202 Active
optical device, 1979. .
Lagerwall "Ferroelectric LCs: The developement of device", Gordon
and Breach Science Publishers S.A., vol. 94, pp. 3-62, 1989. .
Andersson et al. (1989) J. Appl. Physics 66(10):4983-4995. .
Andersson et al. Application of the Soft Mode Ferroelectric Effect
MIeeting Abstract from the Second INternational Conference on FLCs
held in Goeteborg Sweden, 1989. .
Masterson et al., "Ferroelectric liquid crystal tunable filter"
Optics letters vol. 14 No. 22, 1989 pp. 1249-1251..
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Primary Examiner: Miller; Stanley D.
Assistant Examiner: Mai; Huy K.
Attorney, Agent or Firm: Greenlee & Winner
Parent Case Text
This application is a continuation-in-part of U.S. application Ser.
No. 429,304, filed Oct. 30, 1989, now abandoned, which is
incorporated by reference herein.
Claims
We claim:
1. A tunable optical filter comprising one or more stages
positioned in series along a light propagation axis wherein a stage
comprises:
an entrance polarizer which defines a plane of polarization of
light entering said filter stage and an exit polarizer which is
oriented in a fixed position with respect to said entrance
polarizer;
at least one birefringent element and at least one ferroelectric
liquid crystal cell positioned in series along said light
propagation axis between said polarizers wherein said birefringent
element is positioned between said exit polarizer and said
ferroelectric liquid crystal cell; and
means for applying an electric field to said ferroelectric liquid
crystal cell such that said cell is thereby switched between an
unswitched state and a switched state;
wherein said polarizers, said ferroelectric liquid crystal cell and
said birefringent element are oriented with respect to each other
and the plane of polarization of entering light such that, when
said electric field is applied to switch said ferroelectric liquid
crystal cell between said unswitched and said switched state, the
output of said filter is thereby tuned between a first transmission
spectrum defined by the transmission characteristics of said
birefringent element and a second transmission spectrum defined by
the combined transmission characteristics of said birefringent
element and said ferroelectric liquid crystal cell in the switched
state; and wherein, when said filter contains more than one stage,
the exit polarizer of a stage is the entrance polarizer for the
next successive stage positioned in series along said light
propagation axis of said filter.
2. The filter of claim 1 which contains one stage and wherein said
birefringent element is a fixed birefringent element.
3. The filter of claim 1 which contains more than one stage and
wherein said birefringent element is selected from the group
consisting of a fixed birefringent element and a variable
birefringent element.
4. The filter of claim 3 wherein said variable birefringent element
is a ferroelectric liquid crystal cell.
5. The filter of claim 4 wherein said variable birefringent element
is a surface stabilized, ferroelectric liquid crystal cell.
6. The filter of claim 4 wherein said variable birefringent element
is a ferroelectric liquid crystal cell containing a ferroelectric
liquid crystal material which is selected from the group consisting
of a surface stabilized, smectic C* ferroelectric liquid crystal
material, a surface stabilized, smectic A* ferroelectric liquid
crystal material and a distorted helix ferroelectric liquid crystal
material.
7. The filter of claim 1 wherein said ferroelectric liquid crystal
cell is a surface stabilized, ferroelectric liquid crystal
cell.
8. The filter of claim 1 wherein said ferroelectric liquid crystal
cell contains a ferroelectric liquid crystal which is selected from
the group consisting of a surface stabilized, smectic C*
ferroelectric liquid crystal material, a surface stabilized,
smectic A* ferroelectric liquid crystal material and a distorted
helix ferroelectric liquid crystal material.
9. The filter of claim 1 wherein said ferroelectric liquid crystal
cell is a smectic C* ferroelectric liquid crystal cell.
10. The filter of claim 9 wherein said ferroelectric liquid crystal
cell is a surface stabilized, ferroelectric crystal cell.
11. The filter of claim 1 wherein said entrance and exit polarizers
of said stage are oriented parallel to one another.
12. The filter of claim 1 wherein said entrance and exit polarizers
of said stage are oriented perpendicular to one another.
13. The filter of claim 1 wherein said birefringent element is a
fixed birefringent element and is oriented at an angle of
45.degree. with respect to the plane of polarization of light
entering said stage.
14. The filter of claim 1 which contains more than one stage and
wherein each stage comprises at least one ferroelectric liquid
crystal cell and a birefringent element.
15. The filter of claim 14 wherein each stage comprises one
ferroelectric liquid crystal cell and wherein said birefringent
element is a fixed birefringent element.
16. The filter of claim 14 wherein said birefringent element is a
fixed birefringent element.
17. The filter of claim 16 wherein said fixed birefringent element
is oriented at an angle of 45.degree. with respect to the plane of
polarization of light entering said stage.
18. The filter of claim 14 which has a Lyot-type structure.
19. The filter of claim 1 wherein said stage comprises two or more
ferroelectric liquid crystal cells.
20. The filter of claim 19 wherein said ferroelectric liquid
crystal cells are independently switchable between said unswitched
state and said switched state.
21. The filter of claim 20 wherein said ferroelectric liquid
crystal cells are surface stabilized, smectic C* ferroelectric
liquid crystal cells.
22. The filter of claim 19 which contains more than two stages.
23. The filter of claim 22 which as a Lyot-type structure.
24. The filter of claim 1 wherein said polarizers of said stage are
oriented parallel to one another wherein said birefringent element
is a fixed birefringent element oriented at an angle of 45.degree.
with respect to the plane of polarization of light entering said
stage.
25. The filter of claim 24 wherein said stage comprises two or more
ferroelectric liquid crystal cells which are independently
switchable between said switched state and said unswitched
state.
26. The filter of claim 25 wherein said ferroelectric liquid
crystal cells are smectic C* SSFLC cells.
27. The filter of claim 24 which contains more than one stage.
28. The filter of claim 27 which has a Lyot-type structure.
29. The filter of claim 1 wherein said electric field is applied to
said ferroelectric liquid crystal such that the output of said
filter is rapidly tuned between said first transmission spectrum
and said second transmission spectrum.
30. The filter of claim 29 which transmits wavelengths in the
visible spectrum and wherein the output of said filter is tuned
between said transmission spectra at such a rate that the spectra
are detected by a slow response detector as superimposed.
31. The filter of claim 30 wherein the output is detected by the
human eye and the rapid tuning between said output transmission
spectra results in a perceived continuous variation in visible
colors.
32. The filter of claim 31 wherein, when the number of stages is
more than one, the filter has a Lyot-type structure; wherein said
stage contains one birefringent element and one smectic C*
ferroelectric liquid crystal cell; wherein said entrance and exit
polarizers of said stage are oriented parallel to one another;
wherein the birefringent element is a fixed birefringent element
oriented at an angle of 45.degree. to the plane of polarized light
entering said stage, with the thickness of said birefringent
element being selected such that in the unswitched state said
filter transmits a first transmission spectrum which is perceived
by the human eye as a first visible color and the thickness of said
ferroelectric liquid crystal being selected such that in the
switched state said filter transmits a second transmission spectrum
which is perceived by the human eye as a second visible color; and
wherein, when said ferroelectric liquid crystal is switched between
said unswitched and said switched stage, the output of said filter
is tuned between said first transmission spectrum and said second
transmission spectrum such that a continuous variation in colors
which are linear combinations of said first and second visible
colors are perceived.
33. The filter of claim 31 which has one or more stages wherein
when the number of stages is greater than one the filter has a
Lyot-type structure wherein said stage contains one birefringent
element and two smectic C* ferroelectric liquid crystal cells
wherein said entrance and exit polarizers are oriented parallel to
one another; wherein the birefringent element is a fixed
birefringent element oriented at an angle of 45.degree. to the
plane of polarized light entering said stage, with the thickness of
said birefringent element being selected such that in the
unswitched state said filter transmits a first transmission
spectrum which is perceived by the human eye as a first visible
color and the thicknesses of said ferroelectric liquid crystals
being selected such that when one of said ferroelectric liquid
crystal cells is in the switched state said filter transmits a
second transmission spectrum which is perceived by the human eye as
a second visible color and when both of said ferroelectric liquid
crystal cells are in the switched state said filter transmits a
third transmission spectrum which is perceived by the human eye as
a third visible color; and wherein the output of said filter are
tuned between said first, second and third transmission spectra
such that a continuous variation in colors which are linear
combinations of said first, second and third visible colors are
perceived.
34. The filter of claim 33 wherein the thicknesses of said
birefringent element and said ferroelectric liquid crystal cells
are selected so that said visible colors are red, blue and
green.
35. A blocking filter which comprises N stages positioned along a
light propagation axis wherein N [=3 or more along a light
propagation axis] is three or more; wherein a stage comprises:
an entrance polarizer and an exit polarizer which are oriented
either parallel or perpendicular to one another wherein the
entrance polarizer determines the plane of polarization of light
entering said stage;
a ferroelectric liquid crystal cell positioned between said
polarizers such that in its unswitched state the plane defined by
the optic axis of said cell and said light propagation axis is
parallel to said plane of polarization of entering light; wherein
the thickness of said cell and the orientation of said polarizers
are chosen such that when said ferroelectric liquid crystal is in
the switched state said filter transmits a desired wavelength band
while blocking an undesired wavelength band; and
means for applying an electric field to said ferroelectric liquid
crystal cell such that said cell is thereby switched between an
unswitched state and a switched state;
wherein the exit polarizer of a stage in the entrance polarizer for
the next successive state, said N stage filter thereby comprising N
ferroelectric liquid crystal cells; and
wherein said N ferroelectric liquid crystal cells in said different
stages are synchronously switched in pairs of two by application of
said electric field such that when each pair of cells is switched
the filter transmits a selected wavelength band while blocking at
least two undesired wavelength bands.
36. The blocking filter of claim 35 wherein said filter, when said
pair of ferroelectric liquid crystals are in the switched state,
transmits wavelengths within the visible spectrum.
37. The blocking filter of claim 36 wherein said electric field is
applied to said pairs of ferroelectric liquid crystals such that
the output of said filter is rapidly tunable between transmission
spectra.
38. The blocking filter of claim 37 wherein the output of said
filter is switched between said transmission spectra at such a rate
that the spectra are detected by a slow response detector as
superimposed.
39. The blocking filter of claim 38 wherein the output is detected
by the human eye and the rapid tuning between said output
transmission spectra results in a perceived continuous variation in
visible colors.
40. The blocking filter of claim 39 which comprises three
stages:
a first stage comprises a blue blocking surface stabilized smectic
C* ferroelectric liquid crystal cell which is a 3/2.pi. waveplate
at 460 nm bounded by parallel polarizers,
a second stage comprises a green blocking surface stabilized
smectic C* ferroelectric liquid crystal cell which is a 3/2.pi.
waveplate at 550 nm bounded by parallel polarizers and
a third stage comprises a red blocking surface stabilized smectic
C* ferroelectric liquid crystal cell which is a full waveplate at
670 nm which is bounded by perpendicular polarizers;
wherein, when all cells are not in the unswitched state said filter
transmits the unaltered source light, when all cells are in the
switched state said filter transmits no light, when said blue
blocking and said green blocking cells are in the switched state
said filter transmits a red color, when said green blocking and
said red blocking cells are switched said filter transmits a blue
color, and when said blue blocking and said red blocking cells are
in the switched state said filter transmits a green color; and
wherein rapid switching of said blocking cells in pairs results in
a filter output transmission which is perceived as a continuously
variable color output.
Description
FIELD OF THE INVENTION
The present invention relates to tunable optical filters which
employ ferroelectric liquid crystal (hereafter "FLC") materials as
tuning elements and to color generation using such filters.
BACKGROUND OF THE INVENTION
The surface stabilized ferroelectric liquid crystal (SSFLC) light
valve has been shown to possess properties useful in a number of
opto-electronic device applications requiring high contrast ratio.
These include electro-optic shutters, spatial light modulators for
opto-electronic computing, and flat panel display devices. In such
devices, the speed of response is often important. This response
speed is given approximately by equation 1: ##EQU1## wherein .tau.
is the optical response (10%-90%) to an applied voltage step of
magnitude E, .eta. is the orientational viscosity, and P is the
ferroelectric polarization density.
The physics and operation of the SSFLC has been extensively
described (Clark, N. A. et al. (1983) Mol. Cryst. Liq. Cryst.
94:213; Clark and Lagerwall U.S. Pat. No. 4,367,924; Clark and
Lagerwall U.S. Pat. No. 4,563,059). In the surface stabilized
state, FLC molecules lie in layers perpendicular to the glass
plates (the so-called bookshelf geometry). The FLC optic axis makes
an angle .+-..theta. with respect to the layer normal. For many
mixtures, .theta.=.+-.22.5.degree., so the FLC cell acts like a
retarder which can be electronically rotated by 45.degree.. The
voltage requirements for such switching devices are modest (.+-.10
V), and power consumption is quite low because the voltage need not
be applied to maintain the FLC in the switched state: the devices
are bistable (Clark, N. A. and Lagerwall, S. T. (1980) Appl. Phys.
Lett. 36:899). Typical switching times are <44 .mu.s at room
temperature (ZLI-3654 mixture available from E. Merck, D-6100
Darmstadt 1, Frankfurter, Strabe, 250, F.R.G.). Several other
alignment configurations for FLC cells have been described (Clark
and Lagerwall, U.S. Pat. No. 4,563,059).
The contrast (ratio of transmitted light intensity through the cell
in the bright and dark states) in the standard SSFLC cell is
greatest when the tilt angle .theta. of the FLC material is
22.5.degree.. Under these conditions, at the half wave thickness
(where d=.lambda./2.DELTA.n) between crossed polarizers (an
entrance polarizer and an exit polarizer or analyzer) the dark
state will leave polarization of the input light unchanged, while
the bright state will rotate the plane of polarization of the input
light through 90.degree.. In general, in the on (switched) state
the plane of polarization of the output light will be rotated
through 4.theta., where .theta. is the tilt angle.
The orientation viscosity .eta. in FLC mixtures generally increases
with increasing tilt angle. Often, .eta. increases with tilt angle
faster than P, and thus materials with low tilt angle (i.e.
.theta.<15.degree.) often show improved electro-optic response
speed relative to similar materials with 22.5.degree. tilt angle.
However, this increase in speed is achieved at the expense of
throughput, since the output light in the SSFLC is rotated through
<90.degree., and a significant amount of the light in the on
state is extinguished at the analyzer.
Light valves based upon the electroclinic effect occurring in
chiral smectic A* FLC materials exhibit several attractive features
(see Andersson et al. (1987) Appl. Phys. Lett. 51:640), including
very fast response and voltage regulated gray scale. The
electroclinic effect is related to the variation in the
birefringence of a material as a function of an applied electric
field (see Garoff and Meyer (1977) Phys. Rev. Lett. 38:848). A
number of chiral smectic A* materials have been shown to display an
electroclinic effect when incorporated into SSFLC type cells. The
applied voltage induces or varies the tilt angle in these materials
in an analog fashion. The effect is described as being linear in
applied voltage with very rapid response. However, for all
currently known materials, the maximum tilt angle achieved due to
the electroclinic effect is small (i.e.
.theta.<17.5.degree.).
The distorted helix ferroelectric effect has been described with
smectic C* liquid crystals having a short pitch (see Ostrovski and
Chigrinov (1980) Krystallografiya 25:560 and Ostrovski et al. in
Advances in Liquid Crystal research and Application, (l. Bata, ed.)
Pergamon, Oxford; Funfschilling and Schadt (1989) J. Appl. Phys.
66:3877). In SSFLC cells incorporating the shortpitch materials,
the helix of the material is not suppressed, and thus the helix can
be distorted by the application of an electric field. This
distortion results in a field dependent change in the tilt angle of
the material. DHF materials also display voltage-dependent
variations in birefringence. DHF cells are attractive since high
induced tilt angles (up to .+-.38.degree.) can be attained with
applied voltages lower than those required for smectic A* cells.
Beresnev et al., EPO Patent Application published Apr. 5, 1989,
described FLC cells incorporating DHF materials.
Birefringent filters were first used in solar research where
sub-angstrom spectral resolution is required to observe solar
prominences. The first type of birefringent filter was invented by
Lyot (Lyot, B. (1933) Comptes rendus 197:1593) in 1933. The basic
Lyot filter (Yariv, A. and Yeh, P. (1984) Optical Waves in
Crystals, Chapter 5, John Wiley and Sons, New York) can be
decomposed into a series of individual filter stages. Each stage
consists of a birefringent element placed between parallel
polarizers. The exit polarizer for a particular stage acts as the
input (or entrance) polarizer for the following stage. In a
Lyot-type filter, fixed birefringent elements are oriented with
optic axes parallel to the interface and rotated 45.degree. from
the direction of the input polarization. The thickness, and
therefore the retardation of the birefringent elements, increases
geometrically in powers for two of each successive stage in the
Lyot geometry. Multiple stage devices have been demonstrated with
high resolution (0.1 angstrom) and broad free-spectral-range (FSR)
addressing, for example, the entire visible spectrum (Title, A. M.
and Rosenberg, W. J. (1981) Opt. Eng. 20:815).
More recently, research in optical filters has focused on tuning
the wavelength of peak transmission. An optical filter which can be
rapidly tuned has applications in remote sensing, signal
processing, displays and wavelength division demultiplexing.
Tunability of otherwise fixed frequency Lyot filters has been
suggested and implemented using various techniques (Billings, B. H.
(1948) J. Opt. Soc. Am. 37:738; Evans, J. W. (1948) J. Opt. Soc.
Am. 39:229; Title, A. M. and Rosenberg, W. J. (1981) Opt. Eng.
20:815). These include mechanical methods such as stretching
plastic sheets in series with the birefringent elements (Billing's,
B. H. (1948) J. Opt. Soc. Am. 37:738), mechanically rotating
waveplates (Title, A. M. and Rosenberg, W. J. (1981) Opt. Eng.
20:815) or sliding wedge plates (Evans, J. W. (1948) J. Opt. Soc.
Am. 39:229), changing the retardation of the birefringent elements
by temperature tuning the birefringence, or changing the
birefringence using electro-optic modulators (Billings, B. H.
(1948) J. Opt. Soc. Am. 37:738). Temperature tuning and mechanical
tuning methods are inherently slow. Electro-optic tuning of known
filter devices, while potentially more rapid, requires large drive
voltages and is limited in bandwidth by material breakdown voltages
for the thin birefringent elements required (Weis, R. S. and
Gaylord, T. K. (1987) J. Opt. Soc. Am. 4:1720).
Other electronically tunable filters, which have been demonstrated
include acousto-optic tunable filters (hereafter AOTF) (Harris, S.
E. and Wallace, R. W. (1969) J. Opt. Soc. Am. 59:744; Chang, I. C.
(1981) Opt. Eng. 20:824), electro-optic tunable filters (hereafter
EOTF) (Pinnow, D. A. et al. (1979) Appl. Phys. Lett. 34:391;
Lotspeich, J. F. et al. (1981) Opt. Eng. 20:830), multiple-cavity
Fabry-Perot devices (Gunning, W. (1982) Appl. Opt. 21:3129) and
hybrid filters such as the Fabry-Perot electro-optic Solc filter
(Weis, R. S. and Gaylord, T. K. (1987) J. Opt. Soc. Am.
4:1720).
The operation of the AOTF is based on the interaction of light with
a sound wave in a photoelastic medium. Strong acousto-optic
interaction occurs only when the Bragg condition is satisfied.
Therefore, only one spectral component of incident radiation is
diffracted from the structure at a given acoustic frequency. Tuning
is accomplished by changing the acoustic frequency. This was the
first electrically tunable filter, which succeeded in varying the
transmission wavelength from 400 nm to 700 nm by changing the
acoustic frequency from 428 MHz to 990 MHz with a bandwidth of
approximately 80 nm (Harris, S. E. and Wallace, R. W. (1969) J.
Opt. Soc. Am. 59:744). Current AOTF's have 12.degree. fields of
view, high throughput, high resolution and broad tunability (Chang,
I. C. (1981) Opt. Eng. 20:824). However, power requirements are
high for many applications (on the order of 10 watts/cm.sup.2) and
frequency shifts induced by the filter prohibit the use of AOTF's
in laser cavities.
The EOTF consists of a Y-cut LiTaO.sub.3 platelet, placed between
crossed polarizers, with an array of separately addressable finger
electrodes (Pinnow, D. A. et al. (1979) Appl. Phys. Lett. 34:391).
Tunability is accomplished by applying a spatially periodic
(sinusoidal) voltage to the 100 electrodes. Current applications of
this device, however, utilize more elaborate programmable passband
synthesis techniques (Lotspeich, J. F. et al. (1981) Opt. Eng.
20:830). While the power requirements for the EOTF are low, it
suffers from a small clear aperture and field-of-view. This is also
the main disadvantage of the Fabry-Perot devices.
Color switching has been described in liquid crystal displays which
incorporate dichroic dyes (see, e.g., Aftergut et al. U.S. Pat. No.
4,581,608). Buzak in U.S. Pat. No. 4,674,841 refers to a color
filter switchable between three output colors incorporating a
variable retarder which is a twisted nematic liquid crystal cell.
Nematic liquid crystals have also been used for tuning optical
filters (Kay, W. I., U.S. Pat. No. 4,394,069; Tarry, H. A. (1975)
Elect. Lett. 18:47; Gunning, W. (1980) Proc. SPIE 268:190; and Wu,
S. (1989) Appl. Opt. 28:48). The main disadvantage of these is
their slow tuning speed (.about.100 ms).
Clark and Lagerwall in U.S. Pat. No. 4,367,924 "Chiral Smectic C of
H Liquid Crystal Electro-Optical Device" refer to color control as
an attribute of their ferroelectric liquid crystal electro-optical
device and state that "[the] sample birefringence and orientation
of the two polarizers can be manipulated to give color effects." It
appears that the exit polarizers are rotated to select color.
Clark and Lagerwall in U.S. Pat. No. 4,563,059 "Surface Stabilized
Ferroelectric Liquid Crystal Devices" refer to color production
using FLC layers. At least two methods of color production are
discussed. The first involves using spatial multiplexing of a
2.times.2 pixel array containing FLC cells placed between
polarizers to generate four colors where the FLC cells of each
pixel in the array have a different thickness. The second method
involves two FLC layers positioned on top of one another to give
2.times.2 colors. Specifically a device comprised of two FLC
devices which are positioned such that they have a specific tilt
angle of 24.degree. between the optic axes in the switched state is
described for color production.
Ozaki et al. (1985) Jpn. J. Appl. Phys. (part 1) 24 (suppl.
24-3):63 refer to a high speed color switching element in which
dichroic dyes are mixed with ferroelectric liquid crystals. Color
switches and/or displays which combine color filters and
ferroelectric liquid crystal cell shutters have been described.
See, e.g., Seikimura et al. U.S. Pat. No. 4,712,874; Takao et al.
U.S. Pat. No. 4,802,743; Yamazaki et al. U.S. Pat. No. 4,799,776;
Yokono et al. U.S. Pat. No. 4,773,737.
Carrington et al. (1989) Second International Conference on
Ferroelectric Liquid Crystals Program and Abstracts (Goteborg,
Sweden, 27-30 Jun. 1989) Abstract 015 refer to rapid switching of
spatial arrays of FLC two color switches in color displays.
Lagerwall et al. (1989) "Ferroelectric Liquid Crystals: The
Development of Devices" Ferroelectrics 94:3-62 is a recent review
the use of FLC cells in device applications. In a selection called
"SSFLC Color" the reviewers refer to color display (e.g. for
television applications). The reviewers refer to the production of
color using a red-green-blue microfilter repetitive pattern in
front of a liquid crystal and reference J. C. White 91988) Phys.
Tech. 19:91. The reviewers refer to a multicolor FLC screen and
reference Matsumoto et al. (1988) SID 88 Digest 41. The reviewers
also refer to "color sequential backlighting" and reference J. C.
White (1988) supra, and to C. M. Waters (1988) EPO Patent
Application Publication No. 0 261 901.
SUMMARY OF THE INVENTION
The present invention provides discretely tunable and continuously
tunable optical filters which incorporate ferroelectric liquid
crystal (FLC) cells as wavelength tuning elements. Discretely
tunable filters generally will incorporate bistable smectic C* FLC
cells which are SSFLC cells; however, smectic A* FLC cells and
distorted helix ferroelectric (DHF) liquid crystal cells whose tilt
angle changes as a function of the magnitude and sign of the
applied voltage can also be adapted for use in the discrete
filters. Continuously tunable filters comprising FLC cells are
constructed by taking advantage of the field dependent change in
tilt angle of the smectic A* cells or DHF cells, or for certain
applications with slow response detectors by employing the very
rapid switching capability of FLC cells.
Discretely tunable single or multiple stage birefringent filters
are implemented using FLC cells as variable retarders in
combination with an additional birefringent element. A stage of
such a filter, which is defined by polarizer boundaries oriented in
a fixed position with respect to each other, contains at least one
birefringent element, at least one FLC cell and means for applying
an electric field to the FLC cell to induce it to switch from an
unswitched state to a switched state. The FLC cell is oriented such
that in its unswitched state the plane defined by the optic axis of
the cell and the propagation axis is parallel to the plane of
polarization of entering light the stage. The birefringent element
can be a fixed birefringent element whose transmission
characteristics are determined by its thickness or an FLC which is
a variable birefringent element or retarder and which functions as
a birefringent element when it is in its switched state. The
orientation of the fixed birefringent element within the stage can
be varied to obtain a desired filter transmission spectrum,
however, for many applications, the fixed birefringent elements of
these filters will be oriented at an angle of 45.degree. with
respect to the input polarization of light. The entrance and exit
polarizers of a stage of the filter are oriented in a fixed manner
with respect to one another. The angle between the polarizers can
be varied to achieve a desired filter transmission spectrum,
however, for many applications it will be desired to employ
parallel or perpendicular polarizers. Discrete filters can contain
one or more FLC cells in a stage. These FLC cells can be
synchronously switched or independently switched depending on the
application of the filter and/or the desired transmission output.
The FLC cells may have the same thickness or vary in thickness, the
selection of thickness of the FLC cell also depends on the
application of the filter and the desired transmission output.
Since the output of a birefringent element is elliptically
polarized, the FLC cells employed as variable retarders in these
filters must precede the birefringent element. In the case in which
two or more independently switched FLC cells are included within a
single stage of the filter, a switched FLC cell cannot precede an
unswitched FLC cell along the light propagation axis.
The design limitation noted for discretely tunable filters also
apply to the discretely tunable filters which are employed in the
temporally multiplexed continuously tunable filters of the present
invention. Filters which have two or more wavelengths or
transmission spectra are useful for temporal multiplexing. The
driving scheme of the FLC cells is adapted to the desired use of
the filter and the desired transmission spectra.
An embodiment of a discrete filter incorporating FLC cells that is
particularly useful for application to temporal multiplexing is a
wavelength blocking filter. A blocking filter useful for temporal
multiplexing allows switching between at least two spectral
outputs, e.g., two wavelenqths of light. Each stage of the filter
is bounded by polarizers, either parallel or perpendicular
depending on the desired spectral transmission. Each stage contains
a single FLC cell. The single cell may, however, be replaced with
multiple FLC cells that are switched together. The thickness of the
FLC cell in the first stage is selected to block a first undesired
wavelength of light, thus transmitting the unblocked wavelengths.
The filter's second stage contains with a second FLC cell which is
selected to block a second undesired wavelength of light. As such,
when the two FLC cells in the two stages are synchronously
switched, a desired third wavelength is transmitted. A two stage
filter having this configuration switches between transmitting the
output of the source unchanged or no transmission (depending on the
orientation of the polarizers and the choice of FLC cells) and a
selected wavelength band of light that is not blocked by the two
filter stages. A three stage blocking filter can therefore be
switched between three colors, for example, red, green and blue.
Temporal multiplexing of a such a three stage filter can result in
a perception of a wide range of colors by an observer. The
generated colors are linear combinations of the colors switched by
the filter.
The blocking filters are not only useful for selected particular
narrow wavelength bands. The blocking filter can be adapted by
selection of FLC cell thicknesses, by the addition of filter
stages, by the orientation of polarizers and by the application of
different FLC driving schemes to obtain a desired transmission
output. Temporal multiplexing can in general be applied to these
blocking filters to rapidly switch between any such filter
transmission outputs.
The filters of the present invention which incorporate smectic A*
FLC cells or DHF cells are continuously tunable by application of
an electric field over a range of wavelengths defined by the
maximum tilt angle of the ferroelectric material used in the FLC
cell. These filters can contain a single stage or multiple stages
with a stage, as defined by two polarizers at a fixed angle with
respect to one another, containing at least one birefringent
element, an achromatic quarter-wave plate, at least one FLC cell
and a means for applying an electric field to the FLC cell. The
birefringent element and the achromatic quarter-wave plate are
positioned along the light propagation axis between the polarizers
with the birefringent element positioned between the entrance
polarizer and the achromatic quarter-wave plate. The FLC cell is
positioned along the light propagation axis between the achromatic
quarter-wave plate and the exit polarizer and is oriented such that
in its unswitched state the plane defined by the optic axis of the
cell and the light propagation axis is parallel to the plane of
polarization of light entering the stage. The tilt angle of the FLC
material in the FLC cell is dependent on the magnitude and sign of
the applied electric field. When the magnitude and/or sign of the
electric field applied to the cell is changed the transmission
spectrum of the filter is varied. The tuning bandwidth of the
filter will depend on the maximum tilt angle that can be attained
on application of said electric field to the FLC cells in the
filter. Two FLC cells can be cascaded to double the tuning
bandwidth of a filter stage.
The FLC cells of the filters of the present invention are switched
between states by means of application of an electric field. Any
such means that achieves the desired result, i.e., switching, can
be employed. A direct voltage can be applied to the cell or some
form of varying voltage can be applied. An electric field can be
induced by activating a photosensor with light. The applied field
can be electrically or optically induced by any means known in the
art.
Although surface stabilized FLC cells with a bookshelf type
alignment are the most widely used ferroelectric cells, FLC cells
having other types of alignment are known in the art including
those with homeotropic alignment. All such switchable FLC cells can
be applied for use in the filters of the present invention.
A variety of FLC materials, pure compounds and mixtures, are
currently known in the art. Any such mixtures either currently
known or developed in the future can be employed in the FLC cells
of the present invention.
The discretely tunable filters of the present invention are useful
over a wide range of wavelengths ranging from infrared wavelengths
to about 300 nm. The continuously tunable filters of the present
invention which are temporally multiplexed discretely tunable
filters, in principle, are useful over the same wavelength range,
but are continuously tunable only when a slow response detector
(i.e. a detector that averages over many switching cycles of the
filter) is used. These filters are particularly useful in the
visible wavelength region and for applications in which the human
eye is the detector (e.g. color generators, displays). The
continuously tunable filters of the present invention which
incorporate smectic A* and distorted helix FLC cells are generally
useful over a wide spectral range, however, the specific wavelength
region over which they can tune is limited by the maximum tilt
angle that can be achieved by application of an electric field.
Filter stages defined by polarizer boundaries function as
independent units and can be combined to make multiple stage
filters. The exit polarizer for the preceding stage is the entrance
polarizer for the next stage. In a multiple stage filter the ratio
of the thicknesses of all the birefringent elements in a stage
(i.e. FLC cells and fixed birefringent elements) must be the same
in all stages. For example, in the Lyot-type filter structure the
thicknesses of all birefringent elements in sequential stages
increase in the geometric progression: 1,2,4, . . . .
The stages of a discretely tunable filters can, in general, be
combined along a light propagation axis with stages of continuously
tunable filters .
BRIEF DESCRIPTION OF THE FIGURES
FIG. 1 illustrates a single stage of a smectic C* FLC tunable
Lyot-type filter. The net retardation of the stage can be modulated
by electronically rotating the crystal axes [.alpha.(V)] of the FLC
waveplate.
FIG. 2 illustrates a three stage Lyot-type filter incorporating
smectic C* FLC wave plates. This device contains four polarizers
(P1-P4), seven FLC waveplates (LC1-LC7) and three birefringent
elements (B1-B3), which are 1-wave, 2-wave, and 4-wave retarders at
the design wavelength.
FIG. 3, views (a) and (b), compares experimental transmission
(closed circles) of the three stage Lyot-type filter of FIG. 2 with
simulation results (solid line). View (a) compares the measured
transmission spectrum of the three stage Lyot filter, in which the
SSFLC cell is in the unswitched state to simulation results and
view b compares the measured transmission spectrum of the same
filter in which the SSFLC cell is in the switched state.
FIG. 4 illustrates computer simulated superimposed transmission
curves for a 5 stage, 6 channel SSFLC-based tunable filter. The
filter has transmission peaks at 450 nm, 492 nm, 530 nm, 566 nm,
600 nm and 634 nm.
FIG. 5 illustrates an exemplary chromaticity diagram for visible
wavelengths (See Naussau (1983) The Physics and Chemistry of Color,
Wiley Interscience, New York, Chapter 1.) Colors are indicated and
wavelengths are indicated in nanometers (nm) The color
corresponding to standard daylight D.sub.65 is indicated. The
diagram given is generalized and is provided to illustrate that
three colors define a color space.
FIG. 6 illustrates a four-stage, two-channel Lyot-type filter used
to implement temporal multiplexing of FLC cells to achieve
continuously varying visual color generation. P1-P5 are parallel
polarizers which define the four filter stages. B1-B4 are fixed
birefringent elements which are .pi., 2.pi., 4.pi. and 8.pi. wave
plates, respectively, at 540 nm. C1-C4 are FLC cells of varying
optical thickness. The thickness of the FLC layer in cell C1 is 0.6
.mu.m, that of C2 is 1.2 .mu.m, that of C3 is 2.4 .mu.m and that of
C4 is 4.8 .mu.m. The FLC cells in all of the filter stages are
synchronously switched. In the unswitched state the filter transmit
green light (540 nm). In the switched state the filter transmits
red light.
FIG. 7 illustrates the driving schemes employed to obtain visual
color mixing of red and green light in the FLC filter device of
FIG. 6. When the cells are unswitched, the design wavelength is
transmitted (green), view a. When the cells are switched, the
second color (red) is transmitted, view e. When the filter is
switched between transmission of green and red, with each color on
for approximately the same time using the driving scheme of view c,
a yellow color is observed. When the filter is tuned to green for a
higher percentage of the switching period using the driving scheme
of view b, a yellow-green color is observed. When the filter is
tuned to red for a higher percentage of the switching period using
the driving scheme of view d, an orange color is observed. The
colors listed in FIG. 7 are those observed by a subject believed to
have normal color vision.
FIG. 8 illustrates a smectic A* liquid crystal cell with the
molecules arranged in a bookshelf geometry and in the z-y plane of
the containing glass plates. Application of an electric field (E)
switches the molecules form the unperturbed state along the layer
normal (z axis) denoted by n(0), to the tilted state n(E). Tilt
angle is a function of applied field.
FIG. 9 illustrates a single stage smectic A* FLC continuously
tunable filter containing two FLC half-wave plates. The device is
tuned to a desired wavelength by electronically rotating the
optical axis [.alpha.(V)] of the FLC half-wave plate.
FIG. 10, views a-c, compares measured transmission (circles) of the
filter illustrated in FIG. 9 to simulation results (solid lines).
Transmission is shown as a function of wavelength (400-800 nm).
Normalized transmission is indicated along the y axis. The
transmission scale in view (a) is 0 to 1 and in views (b) and (c)
it is 0 to 0.8. View (a) compares experimental and calculated
transmission with the FLC waveplates in the unswitched state. View
b compares experimental and calculated transmission with the FLC
waveplates tuned toward the blue and view c compares experimental
and calculated transmission with the FLC waveplates tuned toward
the red.
FIG. 11 illustrates a computer simulation of the transmission of a
three-stage Lyot filter incorporating smectic A* liquid crystal
half-wave plates. Transmission is shown as a function of wavelength
(480-600 nm). The device has a full width at half maximum (FWHM) of
10 nm with continuous tunability over 70 nm.
DETAILED DESCRIPTION OF THE INVENTION
The present invention provides discretely tunable filters in which
FLC waveplates function as electronically controllable phase
retarders that are incorporated into each stage of a fixed
frequency filter configuration, such as that of a Lyot filter.
Single and multiple stage filters are provided. The addition of
such a retarder to a filter stage, oriented with crystal axis along
the direction of polarization, does not change the spectral
transmission characteristics of the filter. However, rotation of
the FLC waveplate by an appropriate angle is equivalent to
increasing the thickness of the birefringent element. This
effectively changes the design wavelength of the filter (in the FLC
switched state), allowing discrete tuning between wavelengths.
The operation of a discretely tunable birefringent filter using FLC
cells can be understood by analyzing a single filter stage, as
shown in FIG. 1. This single-stage filter contains a fixed
birefringent element and an FLC cell. The direction of propagation
of light is along the z axis of the cartesian coordinate system.
The faces of the birefringent plates and the FLC cells are normal
to the z axis while the electric vector of light transmitted by the
polarizers lies along the y axis. The optic axes of the waveplates
are in the plane normal to the z axis. To describe the operation of
a typical fixed frequency birefringent Lyot-type filter, it is
initially assumed that the optic axis of the FLC is oriented along
the y axis, transmitting the field with no retardation (the
unswitched state). For the case in which the fixed birefringent
element is rotated by 45.degree. about the z axis (the switched
state), the incident linearly polarized light is divided into two
equal amplitude eigenwaves, which travel at different phase
velocities through the birefringent material. The retardation
between the two waves at the exit of the birefringent element is
given by ##EQU2## where n is the birefringence of the material, d
is the material thickness and .lambda. is the free space
wavelength. The two waves interfere at the exit polarizer
(positioned in this case parallel to the entrance polarizer) such
that only wavelengths that are in phase achieve unity transmission.
The transmission spectrum for the n.sup.th stage of a Lyot-type
filter is given by
The transmission of a multiple stage filter is the product of the
intensity transmittances of the individual filter stages. In a
conventional Lyot filter, the thickness of each birefringent
element is always twice that of the previous stage. Each subsequent
stage exhibits a transmission spectrum with half the spectral
period of the previous stage and therefore provides blocking for
the following stage. The transmission spectrum of an N stage filter
can be written in the form of a replicated sin function (Yariv, A.
and Yeh, P. (1984) Optical Waves in Crystals, Chapter 5, John Wiley
and Sons, New York). ##EQU3## The spectral period of the filter, or
FSR, is determined by the stage with the thinnest birefringent
element. The resolution of th filter is determined by the thickest
element. The transmission of a Lyot-type filter (or any other
multiple-stage birefringent filter) does not depend on the order of
the stages, i.e. the stages in the filter need not be ordered by
increasing thickness of birefringent elements.
The transmission spectrum of a single filter stage can be
determined using the 2.times.2 Jones calculus (Jones, R. C. (1941)
J. Opt. Soc. Am. 31:488). These results can easily be extended to a
multiple stage Lyot-type device. The output of the n.sup.th stage
can be represented by the matrix equation
where E.sub.n (.lambda.) and E'.sub.n (.lambda.) are the column
vectors giving the x and y components of the input and transmitted
electric fields, respectively, P.sub.y is the matrix representing
polarizers oriented along the y axis and W.sub.n (.lambda.) is the
matrix for a retarder with crystal axes rotated 45.degree. about
the z axis. These matrices are expressed as (Yariv, A. and Yeh, P.
(1984) Optical Waves in Crystals, Chapter 5, John Wiley and Sons,
New York) ##EQU4## where the retardation, .GAMMA..sub.n (.lambda.),
is given by
Here, .GAMMA..sup.C.sub.n (.lambda.) is the retardation of the
fixed birefringent plate and is given by ##EQU5## where
.lambda..sub.A (=.DELTA.nd) is the design wavelength of the filter
in the unswitched state. This is the wavelength at which the
birefringent element in the first stage is a full-wave plate,
assuming the specific orientation of filter elements shown in FIG.
1. Equation 8 assumes negligible dispersion of the birefringent
elements throughout the tuning range. .GAMMA..sup.C.sub.n
(.lambda.) is the net additional retardation due to the 2.sup.n-1.
FLC's. In the unswitched state, this retardation is zero. In the
switched state (.alpha.=45.degree.), the filter is tuned to a
second wavelength, .lambda..sub.B, due to the additional
retardation. This retardation can therefore be written as ##EQU6##
where .DELTA.n(.lambda.) is the wavelength dependent birefringence
of the FLC's and .DELTA..lambda.=(.lambda..sub.B -.lambda..sub.A).
Due to the highly dispersive nature of liquid crystals, this
expression includes the effect of dispersion of the FLC
birefringence. Using Equations 5 and 6 and the relation
T(.lambda.)=.vertline.E'(.lambda.)/E.sub.y
(.lambda.).vertline..sup.2 yields the intensity transmission given
by Equation 2, where ##EQU7## A model describing the birefringence
of liquid crystals based on a modified version of the Clausius
Mosotti equation of molecular polarizability has been recently
proposed (Wu, S. (1986) Phys. Rev. A 33:1270). This analysis has
shown excellent agreement with experiment and allows us to express
the FLC birefringence, .DELTA.n, as ##EQU8## where G(T) is a
temperature dependent parameter in units of nm.sup.-2, which is a
function of the difference in transition oscillator strengths
between the extraordinary and ordinary directions for light
incident on the liquid crystal molecules, and .lambda.* is the mean
U.V. resonance wavelength. In order to obtain the parameters
required in the above equation the transmission characteristics of
the FLC's placed between parallel and crossed polarizers were
analyzed. Experimentally measured values for these parameters are:
G(T)d=2.08.times.10.sup.-3 nm.sup.-1 and .lambda.*=245.0 nm.
A three-stage Lyot-type filter (FIG. 2) was designed incorporating
SSFLC cells as waveplates (Example 1). Experimental filter
transmission spectra are compared in FIG. 3 with spectra calculated
using the equations presented in the analysis above.
A computer simulation of the filter transmission spectrum of a
six-channel, five-stage Lyot-type filter is shown in FIG. 4 (see
Example 1). The simulated filter contained five FLC cells and a
birefringent element in each stage. The thicknesses of all filter
elements (FLC cells and birefringent elements) increased in the
geometric progression 1, 2, 4, 8, with increasing numbers of
stages. While the order of stages does not affect transmission, the
ratio of thicknesses of the elements within a given stage to the
thickness of the corresponding element in another stage must be
constant. If, for example, the thickness of the birefringent
element in a first stage is 3 times the thickness of that element
in a second stage, then the ratio of thickness of each
corresponding FLC cell in the first stage to the corresponding FLC
cell in the second stage must be 3.
The number of outputs that can be obtained by discrete tuning of a
filter is 1 plus the number of switchable FLC cells in a stage. For
example, a stage containing one birefringent element and one FLC
cell can be switched between two selected transmission spectra. A
stage containing one birefringent element and two FLC cells can be
switched between three transmission spectra.
Filter stages need not contain a fixed birefringent element. The
birefringent element can be replaced by an FLC cell, making it a
variable birefringent element. In this case, in the unswitched
state the filter transmits essentially the transmission spectrum of
the light source entering the filter with no effect on wavelength
(except possibly that due to dispersion).
In cases in which fixed birefringent elements are combined with FLC
cells in a filter stage, the switched FLC cell(s) must precede the
fixed element along the light propagation axis as light exiting the
birefringent element is elliptically polarized.
In cases in which independently switchable FLC cells are combined
in a single stage of a filter, a switched FLC element cannot
precede an unswitched FLC cell along the light propagation axis, as
light exiting a switched FLC cell is elliptically polarized. Thus,
for the case in which two independently switchable FLC cells are
combined in a single filter stage, three transmission outputs can
be obtained: (1) when both FLC cells unswitched; (2) when both FLC
cells are switched and (3) when only the second FLC is
switched.
In multiple stage filters the corresponding FLC cell in each of the
stages must be synchronously switched. Within a stage of a
discretely tunable filter of the present invention, the relative
orientations of the polarizers is fixed, but can be selected to
obtain a desired transmission spectrum. Similarly, while in most
applications the fixed birefringent element will be oriented at an
angle of 45.degree. with respect to the plane of polarization of
light entering a filter stage, this angle can also be selected to
obtain a desired transmission spectrum. The thickness of the
birefringent element and the thicknesses of any FLC cells employed
in the filters are also selected to achieve a desired output
transmission spectrum.
A unique characteristic of FLC cells is their fast switching speeds
(order of 10's to 100's of .mu.sec). Filters of the present
invention are capable of >10 kHz tuning rates, for example,
between two or more discrete wavelengths. In situations where
relatively slow light/color detectors are used, such as with
photographic or movie film, or the human eye, pseudo colors can be
generated using the rapidly switching filters described herein.
Rapid switching between two primary color stimuli can be used to
generate other colors, as perceived by the slow detector, which are
mixtures of the primary colors. For example, the two monochromatic
stimuli, 540 nm (green) and 630 nm (red) can be mixed in various
portions to create the perception of orange (600 nm) and yellow
(580 nm). Optically, this mixing can be done by varying the
quantity of power of the primary stimuli in a transmission. The
same result can be achieved by switching between the two stimuli
(spatially superimposed or closely adjacent) at rates faster than
the response time of the eye (or any detector which averages over
many periods). Colors can be generated in this way using the
filters described herein by varying the time for which the filter
is tuned to any particular primary stimulus compared to another
primary stimulus. By changing the percentage of a square wave
period during which the filter is tuned to one of the primary
stimuli with respect to another (i.e. varying the duty cycle of an
applied voltage, for example), there is a perceived generation of
colors which are mixtures of the primary inputs. In effect, the
quantity of optical power transmitted in each primary stimulus is
varied by changing the ratio of time which the filter is tuned to
each of the primary bands. The response time of the eye is about 50
Hz. The eye will thus average optical power over many cycles of
filter switching, and many colors can be generated for visual
detection.
Color perception by the human eye is actually the result of the
physical wavelength detection by the eye combined with
interpretations of that detection by the brain. Color perception is
often analyzed using a chromaticity diagram like the representative
diagram provided as FIG. 5. In this diagram, the spectral colors
are found along the curved line from violet at 400 nm to red at 700
nm. The diagram indicates a color space that can be accessed on
mixing different amounts of the spectral colors. As suggested by
the shape of the diagram, mixtures employing varying amounts of
three spectral colors (preferably a red, green and blue) will allow
access to the widest range of colors. Specifically referring to the
temporal mixing of the filters of the present invention, changing
the duty cycle or the applied field shifts the color perceived by
the observer, and a filter which switches rapidly between a red,
green and blue output can be used to generate color mixtures which
are linear combinations of those three colors.
A multiple visible color generator employing Lyot-type filters with
fast switching FLC cells is illustrated in FIG. 6. This four-stage
filter was designed (Example 2) to switch rapidly between two
wavelengths (green and red) to visually generate colors which are
linear combinations of the design wavelengths. As seen by reference
to the chromaticity diagram of FIG. 5, colors ranging from red,
orange, yellow through green should be generable. FIG. 7
illustrates the observed visible color output of the filter of FIG.
6 for various pulsing sequences (on cycles of on and off switching)
of the FLC cells. As in all multiple stage filters, the
corresponding FLC cells in all stages are synchronously switched.
For example, a voltage duty cycle which results in the filter being
rapidly switched between red and green, where the time that the
filter transmits red light is about equal to the time the filter
transmits green light, generates a perceived yellow color.
Variations in the duty cycle applied to the filter generate a
continuous range of colors between red and green.
Incorporation of an additional FLC cell in each stage of a filter
like that of FIG. 6 allows temporal switching between three colors
(e.g. red, blue and green). Application of driving schemes
analogous to those used and illustrated with the two color filter
(FIG. 7) results in a visible color generator which can access a
broad area of perceived visible color space.
As a further implementation of the visible color generator
employing rapidly switching FLC cells, the present invention also
provides FLC cell blocking filters.
FLC cells with the required thickness and optical transmission
properties or a particular color generation application can be
readily fabricated using techniques known to the art. Application
of an appropriate voltage duty cycle scheme to switch the desired
pairs of FLC cells can generate a range of perceived colors (color
space), as illustrated in FIG. 5.
In addition a three-stage, three-color blocking filter will
transmit the source light (most often white) with no wavelength
effect in the unswitched state, and will transmit no light in the
fully switched state (black). FLC pulsing schemes of this filter
can include switching to white and black to allow more flexible
selection of generated colors. Blocking filters switching between
two selected wavelengths or more than three selected wavelengths
can be implemented by appropriate selection of FLC cells
(thickness) and positioning of polarizers. Additional spectral
purity of transmitted color (i.e. narrower band width) can be
achieved while retaining blocking of unwanted colors by increasing
the number of stages in the filter with appropriately selected FLC
cells in the stages.
The present invention also provides continuously tunable filters
which do not require temporal multiplexing and are not limited to
use with slow response detectors or to use in the visible spectrum.
These filters utilize smectic A* (SmA*) liquid crystal cells and
DHF liquid crystal cells. The physics and operation of the surface
stabilized SmA* device has been described elsewhere (Clark, N. A.
et al. (1983) Mol. Cryst. and Liq. Cryst. 94:213; and Andersson et
al (1987) Appl. Phys. Lett. 51:640). In the smectic A* phase, the
optic axis is aligned with the layer normal (See FIG. 8). Near the
C*-A* phase transition, the elastic constant approaches zero. This
allows the optic axis to tilt as a linear function of applied
voltage. Placed between crossed polarizers, the device acts like an
analog intensity modulator. The voltage requirement for achieving
the maximum tilt angle of 12.degree.-17.5.degree. for a SmA* device
is modest (.+-.30 V in the A* phase). Typical switching speeds are
.ltoreq.100 ns. Furthermore, a SmA* ferroelectric liquid crystal
tunable filter (continuous FLCTF) can be built with large entrance
apertures, as these cells can be fabricated on large substrates.
Recently described DHF cells will function similarly to the smectic
A* cells in continuously tunable filter configurations of the
present invention. The achievable maximum tilt angles of known DHF
materials (.+-.38.degree.) are significantly larger than those of
smectic A* materials. DHF cells thus will allow wavelength tuning
over wide ranges.
FIG. 9 illustrates the operation of the smectic A* LC tunable
filter (LCTF). The direction of propagation of light is along the z
axis, the faces of the birefringent plates and the LC's are normal
to the z axis, with polarizers oriented along the x axis. Since the
birefringent element is rotated by 45.degree. with respect to the x
axis, the input is divided into two equal amplitude waves, which
travel at different phase velocities through the material. The
retardation between the two waves at the exit of the birefringent
element is given by
where (.DELTA.)n is the birefringence of the material, d is the
material thickness and (.lambda.) is the free space wavelength.
In general, the polarization of broad-band light exiting the
birefringent element is elliptical, with field components parallel
and perpendicular to the direction of the input polarization.
Denoting these field amplitudes, E.sub.x and E.sub.y, respectively,
the ellipticity (E.sub.y /E.sub.x) is a function of wavelength. The
field exiting the birefringent element is incident on the
achromatic quarter-wave plate, which functions as an ellipticity
analyzer (Title, A. M. and W. J. Rosenberg (1981) Opt. Eng.
20:815). This element gives a retardation of .pi./2, independent of
wavelength, bringing the quadrature field components into phase.
Therefore, the achromatic quarter waveplate converts elliptical
polarizations into linear polarizations with wavelength dependent
orientation. The amplitudes of the field components are E.sub.x
(.lambda.)=cos [.GAMMA.(.lambda.)/2] and E.sub.y (.lambda.)=sin
[.GAMMA.(.lambda.)/2], respectively, where .GAMMA.(.lambda.) is
given by Equation 12. Since these two components are in phase, this
represents a linearly polarized field oriented at an angle,
.GAMMA.(.lambda.)/2. Tuning is therefore accomplished by simply
following the achromatic quarter wave plate with a rotatable exit
polarizer, which selects the desired wavelength. In a multiple
state filter this would require rotating every element in
subsequent stages, in order to maintain the desired filter
geometry. Furthermore, this approach requires mechanical rotation
to achieve tuning, which is relatively slow.
A simpler approach that has been described is to introduce a
rotatable achromatic half-wave plate (giving a constant phase delay
of .pi. for all wavelengths) into each stage of the filter (Title
and Rosenberg, supra). A half-wave plate, oriented at an angle
.phi. to a linearly polarized input, simply reflects the linear
polarization about the fast axis of the crystal, giving a rotation
of 2.phi.. Therefore, a rotatable half-wave plate can be oriented
so as to reflect the desired wavelength to the direction of the
exit polarizer. A similar tunable filter can be achieved using the
fast response SmA* or DHF ferroelectric liquid crystal cells.
The transmission spectrum of the tunable color filter, as
illustrated in FIG. 9, can be determined using Jones calculus
(Jones, R. C. (1941) J. Opt. Soc. Am. 31:488). The output of the
filter can be represented by the matrix equation
where E(.lambda.) and E'(.lambda.) are the column vectors giving
the x and y components of the input and transmitted electric
fields, respectively, and P.sub.x and B(.lambda.) are the matrices
representing the polarizers oriented along the x axis and the fixed
birefringent element with crystal axes rotated by 45.degree. from
the x axis, respectively. These matrices are given by Yariv, A. and
P. Yeh (1984) Optical Waves in Crystals, Chapter 5, John Wiley and
Sons, New York: ##EQU9## where the retardation
.GAMMA.(.lambda.)=4.pi..lambda..sub.d /.lambda., and .lambda..sub.d
(=[.DELTA.]nd) is the design wavelength of the filter in the
absence of tuning elements. This is the wavelength at which the
birefringent element functions as a 2(.lambda.) plate. Negligible
dispersion of the fixed birefringent elements is assumed throughout
the tuning range. The matrices ##EQU10## represent the achromatic
.lambda./4 plate and .lambda./2 plate, respectively. In these
expressions, .phi. is the electronically controlled tilt of the
waveplate and .GAMMA..sub.L (.lambda.) is the retardation of the
FLC cell, given by This expression includes the effect of
dispersion of the FLC birefringence, [.DELTA.]n(.lambda.). To
simplify the analysis, it is assumed that the FLC cells function as
perfectly achromatic half-wave plates. However, the computer
simulations take into account the non-achromatic nature of the
FLC's. Assuming perfect achromaticity, Equation 15b can be
rewritten as ##EQU11## Substituting the matrices into Equation 2,
and using the relation T(.lambda.)=.vertline.E'.sub.x
(.lambda.)/E.sub.x (.lambda.).vertline..sup.2, yields the
continuous FLCTF intensity transmittance
Equation 18 gives the selected wavelength .lambda.=.lambda..sub.d
/[1+.phi./.pi.], as a function of angle of the half-wave plate.
High tilt SmA* materials operating near the C*-A* transition have
maximum tilt angles of approximately .+-.12.0. (BDH-76E mixture
available from EM Industries Inc., 5 Skyline Drive, Hawthorne,
N.Y.). Smectic A* materials having tilt angles up to 17.5.degree.
are known. The net tilt angle that can be obtained can be increased
by cascading several FLC cells. Two half-wave plates provide a pure
rotation of twice the angle between their axes. Therefore, two FLC
cells which tilt in opposite directions can provide a maximum net
rotation of 96.degree.. The single stage filter illustrated in FIG.
12 was implemented as described in Example 3. The design wavelength
was set at 540 nm by choice of thickness of a fixed birefringent
element. The smectic A* FLC cells were a half-wave plate at 540 nm.
As demonstrated by the filter transmission spectra of FIG. 13, a
tuning bandwidth of about 115 nm was obtained. Appropriate
application of electric field allows wavelength tuning continuously
within the tuning bandwidth.
Continuously tunable filter stages can be combined to produce
multistage filters in which, for example, enhanced wavelength
resolution can be achieved. Design constraints are as described
above for multiple-stage discretely tunable filters. The
thicknesses of the birefringent elements (both fixed and variable)
within a stage must vary in the same ratio from stage to stage. The
exit polarizer of the preceding stage defines the plane of polarity
of the light entering the next stage. Unlike the discrete filters,
the switched FLC cell in the continuously tunable filter follows
the fixed birefringent in the stage and an achromatic quarter-wave
plate is positioned between the fixed element and the FLC cell. The
fixed birefringent element can also be substituted with a smectic
C* FLC cell (.theta.=45.degree.).
The filter devices described herein above are believed to be the
first continuously tunable FLCTF. Currently, the tunability is
limited by the maximum tilt angles of two LC cells (oppositely
switched). The fundamental tuning range is limited by the spectral
region over which the FLC cells function as half-wave plates. The
continuously tunable FLCTF has potential advantages over other
tunable filters with respect to switching voltages, power
consumption, entrance aperture, field-of-view and switching
speeds.
The present invention has been illustrated by the presentation of a
number of specific embodiments. It is not intended that the scope
of the invention be limited to those embodiments and devices
specifically described.
EXAMPLE 1
A Multiple-Stage Lyot-type Filters Employing SSFLC Wave Plates
A discretely tunable ferroelectric liquid crystal filter was
experimentally demonstrated using the arrangement shown in FIG. 2.
Three birefringent elements, which retard light at 475 nm by one
wave, two waves, and four waves (B1, B2, B3), respectively, were
sandwiched between vertical dichroic sheet polarizers (P1-P4). Each
of these stages in the FLCTF is then modulated by one, two and four
FLC's (LC1-LC7), respectively. These seven FLC devices, fabricated
by Displaytech Inc. (Boulder, Colo.), are half-wave at 400 nm.
The FLC's were actively switched using an HP (Hewlett Packard)
model 8116A function generator. The light source used was a 280 W
tungsten lamp. The filter output was analyzed with a photodiode, an
HP 1726A oscilloscope, and a monochromator.
The experimental results are plotted in FIG. 3 (views a and b)
along with numerical solutions of theoretical curves obtained by
substituting the values for G(T)d and .lambda.* into Equations 1
and 10.
The transmission spectrum of the three-stage Lyot-type filter with
the FLC's in the unswitched state (.alpha.=0.degree.) is shown in
FIG. 3a. Also shown is the theoretical spectrum (solid line). In
FIG. 3b, the spectrum of the Lyot filter with the FLC's in the
switched state (.alpha.=45.degree.) is shown. The transmission is
maximum at 475 nm and 625 nm, which agrees quite well with
theoretical curves (taking into account the FLC dispersion).
The exemplified FLC tunable filter (FLCTF) was not optimized for
maximum transmission and aperture size. However, Lyot filters have
long been considered attractive for these very attributes. High
quality fixed frequency Lyot filters are capable of transmitting
35-40% of incident unpolarized light (Evans, J. W. (1948) J. Opt.
Soc. Am. 39:229). Well known means for optimizing birefringent
filters can be applied to the filters of the present invention.
Additional transmission losses due to absorption, scattering and
Fresnel reflections resulting from the addition of FLC's to a fixed
frequency Lyot filter can be estimated given the transmission of a
single device. This was measured to be typically 0.94 with
broadband AR coating on the substrates. The aperture of the filter
demonstrated was limited by the diameter of the fixed birefringent
elements, as the aperture of the FLC devices was 2.5 cm.
The transmission spectrum of a desired FLCTF can be calculated in a
similar manner to the theoretical curves presented in FIG. 3. FIG.
4 shows the theoretical transmission vs. wavelength curves
superimposed for a six channel, five stage Lyot-type FLCTF which
employs 5 FLC cells in the first stage. These cells give a
retardation of .pi./4 at a wavelength of .lambda.=400 nm. Due to
dispersion in the FLC's, the channels are separated by nearly 50 nm
with an approximate 10-nm bandwidth. As stated above, such
transmission simulations require an experimental determination of
certain transmission characteristics of the FLC cells. For the FLC
cells employed in this Example the experimentally measured values
for these required parameters are: G(T)d=2.08.times.10.sup.-3
nm.sup.-1 and .lambda.*=245.0 nm.
EXAMPLE 2
Continuously Tunable Color Filters Employing Temporal
Multiplexing
A continuously tunable ferroelectric liquid crystal filter using
temporal multiplexing of the FLC cells was experimentally
demonstrated using the arrangement shown in FIG. 6. A four-stage
Lyot-type filter with thicknesses of birefringent elements and FLC
increasing in the ratio of 1, 2, 4 and 8 with stage was contructed
with parallel polarizers defining the stages. The polarizers
employed were HN-22 dichroic sheet polarizers. Four birefringent
elements which retard light at 540 nm by one, two, four and eight
waves (B1, B2, B3 and B4 respectively [FIG. 6]) were placed between
the polarizers (P1-P5). Smectic C* FLC cells (SSFLC's) C1-C4 were
placed in the stages of the filter between the entrance polarizer
and the birefringent element. The birefringent elements are
oriented at 45.degree. with respect to the plane of polarization of
light entering the stage. The FLC cells C1-C4 were constructed to
have specific thicknesses 0.6 .mu.m, 1.2 .mu.m, 2.4 .mu.m and 4.8
.mu.m, respectively to retain the Lyot-structure. The use of FLC
cells of varying thickness rather than multiple cells of the same
thickness in different stages of the filter is preferred as the
filter throughput is significantly increased and the cost and
complexity of the filter is decreased. The resultant filter
switches between red (switched) and green (unswitched).
The FLC cells were switched rapidly as illustrated in FIG. 7.
Application of a - voltage (-vo) switches the FLC cell; application
of the + voltage (+vo) switches the cells to the unswitched state
(green). The light source used was a 280 W tungsten lamp. The
filter output was visually observed by a subject who was believed
to have normal color vision. The various color output can also be
detected photographically.
When the duty cycle of applied voltage was such that the filter
transmitted green light and red light for about the same amount of
time, the subject observed a yellow color (FIG. 7c). When the
filter is tuned to the green for a longer percentage of the
switching period that it is tuned to the red, the subject observed
yellow-green (FIG. 7b). When the filter is tuned to the red for a
longer percentage of the switching period than it is turned to the
green, the subject observed an orange output.
EXAMPLE 3
Continuously Tunable Filters Employing Ferroelectric Liquid Crystal
Materials Which Display the Electroclinic Effect
The SmA* FLC single-stage tunable filter continuous FLCTF shown in
FIG. 9 was experimentally demonstrated. The input and exit
polarizers for the stage (P1,P2) were HN-22 dichroic sheet
polarizers. A birefringent element (B), which retards light at 540
nm by two waves was used as the fixed birefringent plate. SmA*
cells were fabricated to be half-wave plates at 540 nm within .+-.2
nm. The birefringent element and achromatic .lambda./4 plate were
fabricated at Meadowlark Optics (City, State). Two FLC cells
(maximum tilt angle of 12.degree. each) were cascaded in this
filter to increase the maximum tilt angle and expand the tuning
bandwidth.
The FLC cells were switched using a single HP 6299A DC power supply
and temperature controlled to 29.+-.0.2 C.degree.. This temperature
is 1 C.degree. above the C*-A* transition for SmA* BDH764E
electroclinic material used in these experiments (DHC-764E mixture
available from EM Industries Inc., 5 Skyline Drive, Hawthorne,
N.Y.), maximizing .phi.. The light source used was an Oriel model
68735 tungsten lamp. The filter output was analyzed with a
monochrometer with .+-.1 nm resolution and a Newport 820 power
meter.
The experimental results are plotted in FIG. 10 a-c (points) along
with computer simulations (solid lines). FIG. 10a is the
transmission with no field applied, i.e. the design wavelength 540
nm. FIG. 10b is the transmission spectrum for a maximum tilt of
+24.0.degree., i.e. a selected wavelength of 476 nm. FIG. 10c is
the transmission spectrum for a maximum tilt of -24.0.degree., i.e.
a selected wavelength of 623 nm. The experimental tuning bandwidth
of this filter is about 115 nm. The filter can access any
wavelength within this band by appropriate variation of the applied
electric field.
The computer model used to calculate the filter output consists of
a Jones matrix analysis, which takes into account the
non-achromatic nature of the LC half-wave plates using a modified
version of the Clausius Mossotti equation of molecular
polarizability (Wu, S. (1986) Phys. Rev. A. 33:1270). Parameters
required for this model were obtained by analyzing the transmission
characteristics of FLC cells between parallel polarizers. Results
of the model and experiment agree quite well. The discrepancy
between the experimental bandwidth (115 nm) and that predicted in
the ideal case (147 nm) is due to the non-achromaticity of the
.lambda./2 plates.
The computer model was used to calculate the transmission spectrum
of a three-stage Lyot-type filter incorporating continuously
tunable stages. The multiple-stage filter provides higher spectral
resolution with broad and rapid tunability. Results of this
simulation are shown in FIG. 11. The simulated filter has a design
wavelength of 540 nm and incorporates two FLC cells in the first
stage, each having a maximum tilt angle of 12.0.degree., allowing a
tuning range of 70 nm, with a FWHM of 10 nm. FIG. 11 shows the
superposition of three spectra: the design wavelength, the shortest
attainable wavelength, and the longest attainable wavelength. The
filter can address any wavelength within this band.
As noted above, an electroclinic effect has been demonstrated in
SSFLC-type cells incorporating short pitch liquid crystal
materials, distorted helix ferroelectrics. Currently known DHF
materials display maximum tilt angles of about .+-.38.degree.. DHF
electroclinic effect cells have been described, for example, in
Beresnev et al. EPO Patent Application 309,774 (published Apr. 5,
1989). Such DHF cells can be employed in place of or in combination
with smectic A* FLC cells in the continuous filter configurations
described herein.
* * * * *